The conventional gas turbine fuel control is complex electro-mechanical device that uses a number of engine operating conditions (parameters) to regulate fuel flow to the burner to achieve and maintain a commanded engine speed, such as rotor speed N1. The fuel control, using feedback, responds to power lever setting (PLA) to match commanded power and rotor (fan) speed. Among the engine operating parameters that the control typically uses are N1 and N2, respectively the speed of the low and high speed rotors. Other parameters include the temperature and pressure at the inlet and within the compressor stage and exhaust nozzle orientation, in the case of high performance engines employing variable pitch and area exhaust nozzles.
Depending on engine and flight conditions, such a command for peak acceleration from cruise, the control may select one parameter over another on which to "close the loop" for fuel flow to the engine. The transfer function for the control path for each parameter is a so-called proportional integral control, which provides good response and accuracy for aircraft engine applications. The basic transfer for fuel flow WF may be expressed as: EQU WF.sub.t =K1.multidot..intg.WF.sub.Return +K2.multidot..DELTA..delta.t
where WF.sub.t is the total fuel flow at time t. Ideally, the output from each loop (for each engine operating parameter) produces the same scheduled fuel flow (WF.sub.Return) at all times, and if that were true, selecting one loop over another would be invisible in the sense that there would be no immediate change in WF.sub.Return at selection. This is not the case, however, because the parameters have different relationships to engine operation at any instant and thus one may command more or less WF.sub.Return than another at any instant in time, creating a significant stability problem when selecting one channel (loop) over another.
Presently, selection is made between multiple control loops (e.g., N1, N2 and acceleration/deceleration loops) to control a common output. Each loop's response characteristic is defined independently of the other loops by using the current output from the control, i.e., the output for each loop is calculated based on the current control output. The output of all loops are then compared to determine which loop should be selected to produce the control output. Selection is typically based on a series of minimum and maximum selection gates on the output of each control loop. Accordingly, each loop is designed and optimized as a standalone. However, when these loops are configured to work together as a system, using such a selection method, less than optimal control is provided.
One such less than optimal control scenario is found in the interrelationship between the rotor speed (N1, N2) loops and the acceleration/deceleration loops. The rotor speed loops seek to hold a particular engine rotor speed at a particular operating point, such control loops are traditionally designed to be very conservative. The acceleration/deceleration loops seek to take the engine from one power setting to another as fast as possible without exceeding any physical engine operating parameters, whereby such control loops are designed to be very aggressive. It has been found that when a traditional minimum gate is used to select between these control loops, the system will switch off of the acceleration/deceleration loops at a less than optimal time. This results in a transition from one power setting to another that is not as fast as it could have been, had the system continued with the acceleration/deceleration loops for a slightly longer period of time.